This invention relates to radiation detectors and more particularly to scintillation cameras and radioisotope imaging devices.
A scintillation camera having a collimator, a scintillator screen, and photo-multiplier tubes (PMT) coupled to the scintillator was proposed by Hal O. Anger and is described and claimed in U.S. Pat. No. 3,011,057. In the Anger camera the photomultiplier tubes are connected to circuitry which utilizes their their signals to determine the position of each scintillation and to generate light spots or dots on the face of an oscilloscope at corresponding positions.
The Anger PMT circuitry detects both the centroid and pulse height of a gamma ray flash in the scintillator screen. The centroid location is given by x, y coordinates and the brightness or amplitude (or effective pulse height) of the flash is given by z. Therefore, the Anger camera provides the x, y, z representative characteristics of the incident gamma ray. This is done serially, that is each gamma ray flash as represented by this signal pulse at the PMT output is analyzed one by one.
The Anger camera, although widely used, has several basic limitations. These are:
(a) Poor intrinsic spatial resolution (uncertainties in the values of x and y). PA1 (b) Poor pulse height resolution (uncertainties in the values of z). PA1 (c) Poor stability. PA1 (d) Poor count rate.
The basic limitations are more fully discussed below:
(a) Intrinsic spatial resolution. The spatial resolution of the Anger camera has an extrinsic part which is related to external geometrical factors such as the objects distance from the camera, the collimator design, etc. The spatial resolution of the Anger camera also has an intrinsic part which is due to the way a gamma ray loses its energy in the scintillator and the statistics of the division of light photons from each scintillation among the PMTs and the statistics of the generation of the photoelectrons at each PMT. The intrinsic part of lower gamma ray energy levels is almost entirely due to the statistics of the division of light photons and the generations of photoelectrons at the photocathodes of the PMTs. That is, the statistics make the location of the centroid of the flash uncertain. This situation becomes worse as the gamma ray energy decreases. This is why the Anger camera cannot image low energy gamma rays very well. The spatial resolution steadily worsens at gamma ray energy levels below 500 Kev. The use of higher photocathode efficiency PMTs improves the spatial resolution somewhat. The use of more PMTs per camera also improves the spatial resolution, but it is done at the expense of stability and cost of equipment. Current camera's intrinsic spatial resolution at gamma ray energies above 200 Kev. operates at around 5 to 6 mm as measured by the full width at half maximum (FWHM) of the line distribution function, which is only marginally useful for many practical applications in nuclear medicine.
(b) Pulse height resolution. The Anger Camera's pulse height resolution is also marginal such that a large fraction of the unwanted events due to Compton scattered gamma rays are accepted as true signals. This problem worsens at lower gamma ray energies, because the energy separation between the primary gamma ray and Compton scattered gamma rays becomes smaller. The use of higher photocathode efficiency PMTs improves the pulse height resolution somewhat but not enough.
(c) Stability. Stability of the Anger camera is dependent on the gain stability of the PMTs. The more PMTs in each camera, the more control is the problem. Each one percent drift in the PMT voltage supply will cause more than 10% drift in the gain of the PMT.
(d) Count Rate. The count rate capability of the Anger camera in handling larger numbers of events in a short time period is dependent on the decay time of the thallium-activated sodium iodide NaI (T1) scintillator crystal and the dynamic response of the pulse amplifier and the pulse-shaping networks.
In various attempts to overcome one or more of the above-listed limitations of the Anger camera image intensifier tubes were introduced between the scintillator and the photodetectors. Such scintillation camera designs based on the use of image intensifier tubes are numerous and many prototype cameras have been reported. Some reports appeared even before the invention of the Anger camera. Several cameras were made available commercially but none at this day survived in the market place against the universally accepted Anger camera. The failure of these cameras can be attributed to inferior overall performance against the Anger camera. Detailed reviews of this art have been given by Muehllehner (S.P.I.E., Vol. 78, pages 113-117 (1976)) and by Moody, et al (Proc. I.E.E.E., Vol. 58, pages 217-242 (1970)). See also U.S. Pat. Nos. 3,683,185 (Muehllehner) and 3,531,651 (Lieber, et al).
There are several major shortcomings as compared to the Anger camera shared by virtually all such scintillation cameras incorporatng image intensifier tubes. These are:
(1) Poor pulse height statistics such that there is little or no ability for rejecting the Compton scattered events. This generally results in degraded image contrast and poor visibility of cold spots--rendering the camera ineffective in general use. The cause of this is either due to the inability of the design of the camera to provide pulse height analysis or due to poor collection and utilization characteristics of the visible photons from each scintillation flash in the scintillator screen.
(2) Measurable degrees of image distortion such that the camera is not able to provide a high degree of accuracy in the configuration of the image presented. This renders the camera undesirable in studies such as volumetric studies. The cause of this is due to the inherent image distorting in the inverter and minifying type image intensifier tube used and the curved scintillator screen used in the camera.
(3) Noise pulses in Image Intensifier Tubes. Noise sources which are not scintillation in origin are problems common in image intensifier tubes. For low activity gamma ray imaging, this is especially important. A common fault of the cameras in the prior art is the large number of exposed, external, negative high-voltage areas which are potential points of trouble for corona discharge and induced noise pulses.
(4) Bulk and Implosion Hazard and High Voltage Hazard. Bulk is a commonly shared problem. Inherent in the bulk is the large vacuum space enclosed in the image intensifier tubes, which is a potential hazard for implosion and scattered glass fragments.
The high voltage which must be supplied across these image intensifier tubes poses another hazard. One end of these tubes must be operated at high voltage and the other end at ground potential. The high voltage end must be properly insulated so that it will not be a shock hazard. It should also not be a noise source as mentioned above. Frequently the insulation is so thick as the photocathode end that the collimator can not be placed close enough to the scintillator screen to minimize the extrinsic spatial resolution loss for the camera to take advantage of the gained intrinsic spatial resolution.
The closest prior art to the present invention are disclosed in U.S. Pat. No. 3,683,185 (Muehllehner) and U.S. Pat. No. 3,531,651 (Lieber, et al). The Muehllehner camera consists of a flat crystal scintillator screen external to a large diameter image intensifier tube of the electrostatic inverter with minified output design with a curved input photocathode surface, two additional tubes of the electrostatic inverter type design all with a curved input photocathode and curved output phosphor, and a positional sensing detector and circuit. In one of the Muehllehner embodiments and in the Lieber, et al patent are also disclosed designs with a curved scintillator screen inside the image intensifier tube and curved photocathode deposited on the screen. All of these cameras suffer at least the faults discussed at paragraphs 2, 3 and 4 above.
The electrostatic inverter type of image intensifier tubes introduce a substantial amount of spatial distortion making accurate volumetric determination with this camera doubtful. The high voltage must be supplied to the input end making insulation and placement of the collimator difficult. The placement of the scintillator screen outside of the tube causes inefficient optical coupling to the photocathode and in turn causes poor pulse height resolution and poor spatial resolution. Neither Muehllehner, nor Lieber, et al, show how the internal crystal scintillator can be properly used and coupled to the photocathode.